There has been a fair amount of discussion and speculation on when or if Apple could build a Mac computer with a CPU designed around an ARM core. The A-series of CPUs that power the iPhone and iPad are designed around an ARM core, as are the CPUs of virtually all smartphones and tablets and millions of other devices. Most laptop and desktop computers are built around an x86 CPU from Intel or AMD, whether they run Windows, OS X, Linux, or some other OS. The ARM powered Mac discussion seems to center around benchmarking Apple's current A-series CPU against an Intel CPU and pointing out how Apple's designs are inferior to Intel's. However, I think these discussions may be missing the point.
Rather than predicting when an ARM CPU may be powerful enough for a Macbook as we know it, I'd rather speculate on what type of device Apple could build around a state of the art A-series CPU. Could they build a laptop that would perform similar functions to a Chromebook? How about an 'iPad Pro' with more productivity features that could replace a laptop for some people?

An ARM based laptop would have to forego a lot of legacy technology, but Apple has never been shy about doing this. I can imagine a device with only the same I/O as an iPad, meaning no USB or Thunderbolt connectors, video outputs, or SD card readers. The Lightning port could be used for charging and some peripherals, but anything else would be connected over wifi or bluetooth. This device could run apps from the App Store. It wouldn't be able to run OS X apps from the Mac App Store or boot to Windows. It could feature a more usable multitasking interface than iOS does currently. It may not even have a touchscreen. This device could be even thinner and lighter than a Macbook Air and get better battery life. I'd imagine the starting price would be somewhere around $750.

An 'iPad Pro' would be more than just a larger iPad. It could feature native support for keyboards (whether attached as part of an official case or through bluetooth), the ability to view two apps at once, and improved multitasking. It would be used primarily as a tablet with touchscreen input, but the keyboard could be used for typing, navigation, gaming, etc if the user desired. I'd also see this device starting at around $750.

Maybe these two devices could even be the same thing. I believe the point is this: if Apple were to introduce an ARM powered Mac they wouldn't try to cram all of the Macbook's legacy ports, backwards compatibility, etc into it. They'd design a device from the ground up for a specific set of use cases. Just like the iPad is fully capable of replacing a laptop for some small portion of users, an ARM-powered device with native keyboard support could replace a laptop for many more.